概述

读写分离功能作为shardingsphere的可插拔能力之一

包结构分析

api：对外暴露的接口，包括算法spi、配置的对象类
core：核心层，包含默认提供的读分配的负载均衡的算法、以及SQL改写，选取数据源的核心逻辑
distSQL：distsql解析以及管理的相关的流程
spring：spring-boot-starter以及spring-namespace相关的实现。

具体实现

在api层面，对外提供的主要是配置对象以及，读的负载均衡。

在这里，官方提供了两种方式的负载。
1、ROUND_ROBIN（默认算法）
核心实现：

    private static final ConcurrentHashMap<String, AtomicInteger> COUNTS = new ConcurrentHashMap<>();
    
    @Override
    public String getDataSource(final String name, final String writeDataSourceName, final List<String> readDataSourceNames) {
        AtomicInteger count = COUNTS.containsKey(name) ? COUNTS.get(name) : new AtomicInteger(0);
        COUNTS.putIfAbsent(name, count);
        count.compareAndSet(readDataSourceNames.size(), 0);
        return readDataSourceNames.get(Math.abs(count.getAndIncrement()) % readDataSourceNames.size());
    }

2、RANDOM

    @Override
    public String getDataSource(final String name, final String writeDataSourceName, final List<String> readDataSourceNames) {
        return readDataSourceNames.get(ThreadLocalRandom.current().nextInt(readDataSourceNames.size()));
    }

核心路由逻辑

通过下述逻辑获取对应的读或者写的数据源。

/**
     * Route.
     * 
     * @param sqlStatement SQL statement
     * @return data source name
     */
    public String route(final SQLStatement sqlStatement) {
        if (isPrimaryRoute(sqlStatement)) {
            String autoAwareDataSourceName = rule.getAutoAwareDataSourceName();
            if (Strings.isNullOrEmpty(autoAwareDataSourceName)) {
                return rule.getWriteDataSourceName();
            }
            Optional<DataSourceNameAware> dataSourceNameAware = DataSourceNameAwareFactory.getInstance().getDataSourceNameAware();
            if (dataSourceNameAware.isPresent()) {
                return dataSourceNameAware.get().getPrimaryDataSourceName(autoAwareDataSourceName);
            }
        }
        String autoAwareDataSourceName = rule.getAutoAwareDataSourceName();
        if (Strings.isNullOrEmpty(autoAwareDataSourceName)) {
            return rule.getLoadBalancer().getDataSource(rule.getName(), rule.getWriteDataSourceName(), rule.getReadDataSourceNames());
        }
        Optional<DataSourceNameAware> dataSourceNameAware = DataSourceNameAwareFactory.getInstance().getDataSourceNameAware();
        if (dataSourceNameAware.isPresent()) {
            Collection<String> replicaDataSourceNames = dataSourceNameAware.get().getReplicaDataSourceNames(autoAwareDataSourceName);
            return rule.getLoadBalancer().getDataSource(rule.getName(), rule.getWriteDataSourceName(), new ArrayList<>(replicaDataSourceNames));
        }
        return rule.getLoadBalancer().getDataSource(rule.getName(), rule.getWriteDataSourceName(), rule.getReadDataSourceNames());
    }
    
 private boolean isPrimaryRoute(final SQLStatement sqlStatement) {
        return containsLockSegment(sqlStatement) || !(sqlStatement instanceof SelectStatement) || HintManager.isWriteRouteOnly() || TransactionHolder.isTransaction();
    }